The present invention relates to an access circuit of an optical disk unit, to prevent any access error caused by the reduction of reflected light from a light spot projected to an optical disk due to the influence of pits existing on the optical disk.
In the access circuit of the conventional optical disk unit, the moving direction of the light spot is detected to accurately position the light spot on an object track. Heretofore, a tracking signal and total reflected light amount signal have been used for this detection of the moving direction of the light spot.
The tracking signal is formed as follows. The reflected light of a light spot projected onto an optical disk is incident through a converging lens upon a pair of horizontally symmetrical photo sensors, and the difference between the output signals from the photo sensors in this pair is taken as the tracking signal. The tracking signal becomes zero when the light spot takes a position on a track or on a guide recess in the optical disk.
The total reflected light amount signal is formed as follows. The reflected light of the light spot projected onto the optical disk is incident through the converging lens upon the pair of horizontally symmetrical photo sensors, and the output signals from the photo sensors in this pair are added to each other. Thus, the total reflected light amount signal is generated. This total reflected light amount signal takes the maximum level when the light spot is in a position on a track, while it takes the minimum level when the light spot is on a guide recess in the optical disk.
FIG. 1(a) shows the status in which a light spot S is projected onto a surface 11 of an optical disk 10 and moving at a predetermined speed in the direction of arrow A. As shown, there are on the surface 11 of the optical disk 10 a plurality of regularly spaced guide recesses 13, and a track 14 in the center between the two successive guide recesses 13. The reference numerals 13a and 13b indicate the ends of the guide recesses 13.
FIG. 1 (b) shows one example of a tracking signal TS' delivered when the light spot S moves at the predetermined speed as shown in FIG. 1(a). In FIG. 1(b), the vertical axis takes a voltage value while the horizontal axis takes a time. It should be noted here that the time interval t1 in FIG. 1(b) is equal to a time which the light spot S takes when it moves over a distance 1 (between the two successive guide recesses 13) shown in FIG. 1(a) at the determined speed. As apparent from FIGS. 1(a) and (b), the tracking signal TS' becomes zero when the light spot S takes a position on the track 14 and also when it is on the guide recess 13. The reason is as follows. That is, when the light spot S takes a position on the track 14, and also when it is on the guide recess 13, the amounts of reflected light incident upon the pair of photo sensors in pair are equal to each other.
FIG. 1(c) shows one example of a total reflected light amount signal RS' delivered when the light spot moves as shown in FIG. 1(a). In FIG. 1(c), the vertical axis indicates a voltage value while the horizontal axis takes a time. Note that the time interval t1 shown in FIG. 1(c) is equal to a time which the light spot S takes when it moves over a distance 1 (between the two successive guide recesses 13) shown in FIG. 1(a) at the predetermined speed. As seen from FIGS. 1(a) and (c), the total reflected light amount signal RS' takes the maximum level when the light spot S is on the track 14 while it takes the minimum level when the light spot S is on the guide recess 13. The reason is as follows. Namely, when the light spot S is on the track 14 (at the intermediate position between the guide recesses 13), the reflected light is less scattered so that the light reflection becomes the largest. On the contrary, when the light spot S takes a position on the guide recess 13, the reflected light is scattered by the guide recess 13 with the result that the reflection is minimized.
Referring now to FIGS. 2(a) thru (g), how the moving direction of the light spot is detected will be described. FIG. 2(a) shows one example of the tracking signal TS', and FIG. 2(b) shows one example of the total reflected light amount signal RS'. The signal 21 shown in FIG. 2(c) is a a signal derived from zero level-based binary-coding of the tracking signal TS' shown in FIG. 2(a). Also the signal 22 shown in FIG. 2(d) is a signal derived from binary-coding based on slice level SL of the total reflected light amount signal RS' shown in FIG. 2(b). It should be noted here that the trailing edge of the signal 22 shown in FIG. 2(d) is a signal indicative of the end 13a (see FIG. 1(a)) of the guide recess 13. To detect the moving direction of the light spot S, whether the signal 21 is H or L is determined at the trailing edge of the signal 22 that indicates the end 13a of the guide recess 13. Thereby, it is possible to determine whether the optical head (not sown) is positioned at the right or left side of the end 13a of the guide recess 13. The signal 23 shown in FIG. 2(e) is a pulse signal generated synchronously with the trailing edge of the signal 22 shown in FIG. 2(d). Therefore, by determining at the delivery of the pulse signal 23 whether the signal 21 is H or L, the moving direction of the light spot can be known. FIG. 2(f) shows the detection, as signal 24, of the light spot S moving from the left to right, and FIG. 2(g) shows the detection, as signal 25, of the light spot S moving from the right to left.
In an optical disk unit in which information is recorded by forming pits in the optical disk, the total reflected light amount signal TS' takes a waveform attenuated at the point where a pit P is formed in the track 14, as shown with dot line in FIG. 1(c). This is because the reflected amount of light from the optical disk is reduced as influenced by the pit P formed in the disk with the result that the amplitude of the total reflected light amount signal RS' is decreased.
As described in the above, if the amplitude of the total reflected light amount signal RS' is made small due to the existence of a pit, the trailing edge of the signal 22 shown in FIG. 2(d) cannot accurately indicate the end 13a of the guide recess 13 in the optical disk 14. As a result, the moving direction of the light spot S cannot be correctly detected. Note that the optical disk unit in which information recording is done by forming pits includes, for a write-once optical disk unit.
The previously-mentioned conventional techniques include, for example, the invention disclosed in the Japanese Unexamined Patent Publication (kokai) No. 58-91536.